5 The Badland Formations of the Black; Hills Kegion 



■ 

 grit of a distinctly conglomerate nature, occasional pebbles, 

 ranging up to nearly one inch in diameter. This dike where 

 observed, projects in places twelve to eighteen inches above the 

 general surface, is six to eight inches thick and is exposed in 

 a straight line for a distance of several rods. The general nature 

 of this dike is the same as in the other dikes described, but the 

 conglomerate character so far as I am aware, has not been 

 recorded elsewhere in the Badlands. A view of this dike is 

 given on Plate 19. 



It is interesting in this connection to note that dikes similar 

 to those in the badland formations have been observed in the 

 Cretaceous shales near the main Black Hills uplift, and Mr. 

 Darton in describing them indicates that the material came from 

 below, and that the dikes were not all formed at the same time 

 and that at some points the shale on either side of the dike is 

 deflected abruptly downward for a few inches.* 



GEODES 



Geodes are spheroidal masses of mineral matter formed by 

 deposition of crystals from some mineral solution on the walls of 

 -6 rock cavity. The growth is constantly inward toward the 

 center. If the process of deposition has continued sufficiently 

 long, the crystals reach across the depositional space, interlock 

 with each other and the geode becomes solid. Often the crystals 

 project only part way, leaving a considerable cavity and then 

 the geode when broken presents a crystal lining of much beauty 

 and interest. Commonly the geodes are more or less siliceous, 

 especially in the outer portions and, resisting weathering better 

 than the enclosing rock mass, may often be found freed from the 

 matrix lying on the disintegrating surface. Not infrequently 

 crystal fragments become detached within the shell, and these, 

 striking against the inner walls when the geode is shaken, serve 

 to make a sound. For this reason the geodes are often referred 

 to locally as rattle stones. 



Many geodes have been collected from the Big Badlands, 

 but little is known of their occurrence or origin. The diameter 

 varies from one inch or less to several inches. The prettiest ones 

 of rather small size are said to be found near Imlay. Much 

 larger ones occur near the mouth of Medicine Rod creek. 

 Those that have come under my observation have commonly an 

 irregular shell of chalcedony more or less filled with bright clear 



*Edgemont Polio, p. 5, column 4. 



